This invention relates to plated metal articles for protecting a metal substrate from corrosion and, in particular, to an article having a multilayer finish comprising an amorphous metal underlayer, a corrosion-resistant metal middle layer and one or more precious metal outer layers. The surface finish is especially useful for electrical and electronic devices such as connectors and integrated circuit lead frames.
Corrosion resistance is particularly important in electrical and electronic devices including electrical connectors. High quality connectors are increasingly important in a wide variety of products including consumer electronics, household appliances, computers, automobiles, telecommunications, robotics and military equipment. Connectors provide the paths whereby electrical current flows from one device to another. Quality connectors should be highly conductive, corrosion resistant, wear resistant, readily connected by solder and preferably inexpensive.
Unfortunately no single material has all of the desired characteristics. Copper and many of its alloys are highly conductive, but they are subject to corrosion in typical ambients, producing reactive oxides and sulfides. The corrosion products on the surface reduce the conductivity of the connectors and the reliability of interconnection. The reactive corrosion products also interfere with the formation and reliability of solder bonds and can migrate to other electronic components which they adversely affect.
Thin precious metal layers have been applied to copper surfaces to increase corrosion resistance. But the precious metal layers are porous, and corrosion through the pores continues to be a problem.
Nickel coatings have been applied under the precious metal outer layer to serve as barriers to the formation and migration of reactive copper products. However nickel coatings also exhibit porosity. Although the porosity and corrosion are reduced by the nickel barrier, the corrosion products of copper can still form and migrate through the finish. The porosity of the nickel coatings can be reduced by increasing the nickel thickness, but thick nickel coatings take substantially longer to produce and tend to crack in use. Longer production times mean lower productivity and higher production cost.
An improved integrated circuit lead frame comprising copper coated with nickel followed by a composite of noble metal layers is described in U.S. Pat. No. 5,675,177 issued to applicant J. A. Abys et al. on Oct. 7, 1997, which is incorporated herein by reference. These coated lead frames have high corrosion resistance and good electrical and mechanical properties. However even these surface finishes cannot prevent the substrate metal from corrosion in a corrosive environment. Accordingly, there is a need for a surface finish with improved porosity reduction.
In accordance with the invention, a metal substrate is coated with a multilayer surface finish comprising, in succession, an amorphous metal underlayer, a corrosion-resistent metal middle layer and one or more outer layers of precious metal. In an exemplary embodiment the metal substrate comprises copper alloy, the amorphous metal underlayer is Ni-P, the middle layer is nickel and the outer layer is palladium. The resulting structure is particularly useful as an electrical connector.